It is known to expand polymers, following a physical method; a liquefied gas is added to the molten resin and the evaporation of said liquified gas (at the outlet of an extruder's orifice) gives rise to a cellular structure. The foam materials obtained until now are not free from shortcomings, like for instance an unacceptable shrinkage and a change of density with time, namely a poor dimensional stability. To suppress such drawbacks, a particular blowing agent was proposed, the 1-2-dichloro-tetrafluoro-ethane (known also as ALGOFRENE 114) suggested for instance by Japanese Patent Publication 60/4341. Also by this way, however, it isn't possible to eliminate completely such drawbacks and to obtain high quality foam materials. That is true especially for the surface smoothness of low-density materials (.rho.&lt;100 Kg/m.sup.3).
Recent papers described the extrusion expansion of polyolefinic compositions containing special additives for the control of the dimensional stability, in particular additives comprising fatty acid amides, the corresponding amines and the esters of the same acids (U.S. Pat. No. 4,214,054), as well as the semiesters of said acids with polyols (U.S. Pat. Nos. 3,644,230 and 3,755,208). We remember, moreover, the aliphatic esters and semiesters of U.S. Pat. No. 4,345,041, as well as the corresponding technologies (see Japanese Patent Publications 78/102971, 79/34374 and 79/39467). Unfortunately, such additives involve some shortcomings, if flexible foam materials of very low density are required (20-40 Kg/cm.sup.3). The poor compatibility with polyolefins in fact gives rise to diffusion phenomena after the expansions and the foam materials, therefore, display a poor stability when in contact with metals (in the presence of moisture), because of corrosion. This shows unsatisfactory features for the thermal and electric insulation (this is true e.g. in the case of a cable jackets and insulations). These methods, moreover, are requiring non neglectable consumption of blowing agent.
The expansion of polyolefines by chemical blowing agents, e.g. azodicarbonamide (H.sub.2 N--OOC--N.dbd.N--COONH.sub.2), was also already described. In this case, a cross linking occurs, under formation of transversal --C--C-- bonds between the polyolefinic chains, realized e.g. by means of peroxides. See hereinafter formula (I). Such cross-linking permits the utilization of a wider range of visco-elasticity for the polymer, during the expansion step.
The increase in viscosity of the molten polymer, however, consequent to said cross-linking, does not permit, in this case, a normal extrusion. The expansion, therefore, must occur successively or contemporaneously to the cross-linking and not before that. For the manufacture on industrial scale of high quality foam materials, until now, only chemical blowing agents were used, which permit compact (not expanded) semifinished products, which then undergo a cross-linking and a thermal expansion caused by the gases released by decomposition of the blowing agents during an expansion molding. Such methods require a heavy immobilization and don't permit low-density foam materials to be obtained (in particular when .rho.=25-50 Kg/m.sup.3), like the materials bound to the packaging of food and to the building field, where usually are requested, in view of the thermal insulation, also suitable fire-extinguishing features (see e.g. European Patent No. 144,015, in the name of the Applicant).
The Applicant has now found an extrusion expansion process, using physical blowing agents, for compositions based on copolymers of ethylene with vinyl acetate, which permits, also in the absence of additives for the dimensional stability control, to obtain outstanding soft and low-density materials (with fair dimensional stability and fair superficial smoothness, free from creases on the surface and of cracks on the cell walls, with high compression strength) suitable for the thermal or electrical insulation as well as for the packing field. Such process permits, moreover, to lower the consumption of halogenated propellant, as well as an increase in the extruder output capacity.